1. Field of the Invention
The present invention relates to plasma display panels. More particularly, the present invention relates to plasma display panels of the matrix variety containing a plurality of individual display cells defined by the intersection of substantially orthogonal sets of conductors. Still more particularly, the present invention relates to such matrix plasma panels employing so-called "keep-alive" cells disposed around the periphery of the matrix proper for purposes of facilitating the breakdown of the gas at an addressed cell by increasing the density of photons, photo electrons and ions at that cell.
2. Description of the Prior Art
In U.S. Pat. No. 3,559,190 issued Jan. 26, 1971, to D. L. Bitzer et al, there is disclosed a gaseous display and memory system which may be characterized as being of the pulsing discharge type having a gaseous medium, usually a mixture of two gases at a relatively high pressure, in a thin gas chamber or space between opposed dielectric charge storage members which are backed by conductor arrays. The conductor arrays backing each dielectric member are typically arranged in overlapping orthogonal manner to define a plurality of discrete discharge volumes or cells. The discharge units in the Bitzer et al system are additionally defined by a perforated plate interposed between the two dielectric members with the perforations being aligned at points where the overlapping of the conductor arrays occur. In U.S. Pat. No. 3,499,167 issued Mar. 3, 1970 to Baker et al, a similar system is disclosed. Because of other system parameters, it is possible to eliminate in the Baker et al system the physical barriers provided by the perforated member.
In any event, in plasma panels of this general type operation is based on the fact that a conducting plasma of electrons and positive ions is produced upon ionization of the gas contained in the envelope of the panel. This occurs upon selection of a particular cell by applying appropriate operating potentials to a particular pair of crossed conductors, one from each of the orthogonally arranged arrays. When a cell is once selected by standard half select techniques and a gas discharge is effected at a particular selected cell, it is possible to maintain in future cycles the discharge at that cell with a somewhat lower operating voltage. Thus, though a particularly high voltage is necessary to write such a cell, it proves possible to sustain a discharge at subsequent times by repetitively applying an AC (sinusoidal or pulse) sustain signal having a magnitude lower than the write signal.
A description of typical commercially available AC plasma display system, is contained in Johnson and Schmersal, "A Quarter-Million-Element AC Plasma Display with Memory," Proc. of the S.I.D., Vol. 13, No. 1, First Quarter 1972, pp. 56-60. The panel described in the Johnson and Schmersal paper is manufactured by Owens-Illinois, Inc.
It is well known in the art that to facilitate the operation of cells disposed in a matrix fashion on a substantially planar panel, e.g., of the type described in the Johnson and Schmersal paper, the working atmosphere surrounding each cell is advantageously "enriched" by the presence of free ions, electrons or photons. It has proven advantageous in prior art systems to provide an initial source of such ions or photons integral with the panel itself, or to apply photons from an external source, e.g., from an ultraviolet light source. In providing a source of ions or photons by virtue of structure integral with the matrix display proper, it has proven useful to provide so-called "keep-alive" cells which have as their purpose to create the required ions or photons. Such keep-alive cells are described, for example, in U.S. Pat. No. 3,654,507 issued Apr. 4, 1972 to Caras and Ogle; and in Holz, "The Primed Gas Discharge Cell--A Cost and Capability Improvement for Gas Discharge Matrix Displays," Proc. of the S.I.D., Vol. 13, No. 1, First Quarter 1972, pp. 2-5.
The above-cited panel described in the Johnson and Schmersal paper advantageously utilizes such keep-alive cells as well; the panel described and manufactured by Owens-Illinois utilizes keep-alive cells positioned around the entire panel. In typical configuration, then, the band of cells, including four rows or columns of cells around the borders of the Owens-Illinois panel, are maintained in the "on" state to create the required radiation (photons or photo electrons, etc.). These border keep-alive cells are driven from a separate sustain source which is adjusted to synchronize with the display sustain signals unless some cell is to be addressed. When a cell is to be addressed, the keep-alive sustain signal temporarily resynchronizes with the addressing signal, typically preceding it by 1-2 .mu. sec instead of by the 6 .mu. sec it would if resynchronization did not take place.
It has been the experience of plasma panel designers, especially those desiring to build a panel of any substantial size, e.g. comprising a 512 .times. 512 matrix of cells, that there is a considerable variation in the threshold for signals to accomplish the writing of information. This variation is related, in part, to the position on the panel of a cell being selected. Thus, in particular, in the Owens-Illinois panels it has been a common experience that cells centrally located on the panel have, in general, a higher threshold for writing. This may be explained in part by the fact that such cells are especially remote from the border "keep-alive" cells, and therefore from a ready source of radiation, photoelectrons and other ions.
While uniformity in writing voltages alone is desirable, it should be understood that any lack in this regard is not a matter of mere inconvenience. Thus, if a write voltage level is selected which is unusually high, so that it is sure that it will be sufficient for all cells in a matrix array, care must be exercised that spurious operation of non-selected cells is avoided. Thus, in recognizing that one must require at least a minimum threshold value while not exceeding a maximum value (to avoid crosstalk), it is clear that there exists a range of acceptable values for the write signals in a plasma panel display. Because not all panels manufactured have identical physical characteristics, (e.g., spacing between dielectric planes, aging characteristics, and the like), it is required that some allowable variability of voltage for the write signals be present. Thus, there must be an operating range or margin for such write signals to account for variability in particular panel characteristics. In addition to panel-to-panel variations, it will be understood that cell-to-cell variations for a given panel will also occur.
It is therefore an object of the present invention to provide an apparatus and method for improving the uniformity of voltages required to write information into (or otherwise address) a selected cell in a plasma panel matrix while maximizing the addressing voltage margins for the panel.
A partial fulfillment of these objectives is achieved by the dynamic keep-alive scheme presented in my copending application, Ser. No. 460,757 filed Apr. 15, 1974, now U.S. Pat. No. 3,979,638 issued on Sept. 7, 1976. In a typical embodiment disclosed there, the leading edge of the keep-alive sustain pulse is stretched to precede the addressing signal by a time increment dependent on the distance between the addressed cell and the nearest keep-alive border. Thus cells which are spatially far removed from the keep-alive borders experience the effects of keep-alive priming almost simultaneously with the beginning of the addressing signals, while there is a considerable time lag between the priming and the addressing of cells close to the keep-alive borders. The net effect is to make a uniform effective quantity of priming radiation available to each cell at the time it is addressed, in general permitting more uniform operating voltages and wider operating margins over the panel.
However, the time lag necessary to produce uniform priming in even a moderate-sized display is often considerable, up to 6 .mu. sec in some 512 .times. 512 panels, for example. In addition, the considerably stretched keep-alive sustain pulse required may give rise to a number of problems which will later be explained in detail.
Further objects of this invention therefore include refining the control over operating margins afforded by dynamic keep-alive technique while minimizing the sustain pulse distortion necessary to produce these results.